The long term objective of this study is to understand the mechanism of activation of the platelet integrin GPIIb-IIIa. GPIIb-IIIa is key to platelet adhesion and aggregation. An understanding of its regulation is likely to be important for treating many cardiovascular diseases including myocardial infarction and stroke. GPIIb-IIIa exists on resting platelets in a dormant conformation unable to bind soluble fibrinogen. Upon platelet stimulation, IIb-IIIa becomes capable of binding fibrinogen and mediating platelet aggregation. Although activation of IIb-IIIa is key to platelet function, the mechanism its activation has not been solved. One hypothesis of the study is that proteolytic cleavage of the cytoplasmic domains of IIb-IIIa control the activation state of the integrin. This hypothesis will be tested by characterizing the structural differences in the cytoplasmic domain of two purified forms of IIb-IIIa which differ in activation state. A second goals of the study is to understand the structural basis of activation-dependent ligand binding to IIb-IIIa. Phage-display will be used to select ligands that bind preferentially to the dormant and active forms of the integrin. Results from this study are likely to provide a structure-activity series explaining activation-dependent ligand binding. A third aim of the study is to understand the kinetic aspects of integrin activation. Does activation result from an increase in ligand association rate or a decrease in ligand dissociation rate? These studies will be performed on whole platelets and with purified forms of dormant and active IIb-IIIa. A final goal is to understand how the divalent ion binding sites on IIb- IIIa influence the activation event. Binding studies will be performed between Ca2+ and purified conformers of dormant and active IIb-IIIa. Results from this analysis should determine which class of ion binding sites regulate activation of the integrin.